1 + 2 Normal
Insulator nose grayish-yellow to russet brown.
Engine is in order. Heat range of plug correct.
Mixture setting & ignition timing are correct, no misfiring, cold starting device functions correctly.
No deposits from fuel additives containing lead or from alloying constituents in the engine oil.
No overheating.
3 + 4 Soot - carbon fouled
Insulator nose, electrodes & spark-plug shell covered with velvet-like, dull black soot deposits.
Cause : Incorrect mixture setting (carburetor, fuel injection) mixture too rich, air filter very dirty, automatic choke not in order or manual choke pulled too long, mainly short distance driving, spark plug too cold, heat range code number too low
Effect : Misfiring, poor cold-starting performance.
Remedy : Set mixture & cold-starting device correctly, check air filter.
5 + 6 Oil-fouled
Insulator nose electrodes & spark-plug shell covered with shiny soot or carbon residue.
Cause : Too much oil in combustion chamber. Oil level too high, badly worn piston rings, cylinders & valve guides. In two stroke gasoline engines, too much oil in mixture.
Effect : Misfiring, poor starting performance.
Remedy : Overhaul engine, correct fuel-oil mixture, new spark plugs.
7 + 8 Lead deposits
In places the insulator nose is glazed brownish yellow; this may also tend towards green.
Cause : Fuel additives which contain lead. The glaze appears in the case of heavy engine loading after lengthy
part load operation.
part load operation.
Effect : Under heavy loading deposits become electrically conductive & cause misfiring.
Remedy : New spark plugs, cleaning is pointless.
9 + 10 Heavy Lead deposits
In places the insulator nose is thickly glazed brownish-yellow, this may also tend towards green.
Cause : fuel additives which contain lead. The glaze appears in the case of heavy engine loading after lengthy
part load operation.
part load operation.
Effect : Under heavy loading, deposits become electrically conductive & cause misfiring.
Remedy : new spark plugs, cleaning is pointless.
11 + 12 Heavy Lead deposits
Heavy ash deposits from oil & fuel additives on the insulator nose, in the scavenging area & on the ground electrode. The structure of the ash is loose to cinder-like.
Cause : Alloying constituents, in particular from oil, can deposit this ash in the combustion chamber & on the spark-plug face.
Effect : Can lead to auto ignition with loss of power & engine damage.
Remedy : Repair engine. New spark plugs. Possibly use other oil.
13 Partially melted centre electrode
Centre electrode partially melted, blistered, spongy insulator tip.
Cause : Overheating due to auto ignition, eg: due to
over advanced ignition timing, combustion deposits in combustion chamber, defective valves, defective ignition distributor, inadequate fuel quality, heat range possibly too low.
over advanced ignition timing, combustion deposits in combustion chamber, defective valves, defective ignition distributor, inadequate fuel quality, heat range possibly too low.
Effect : Misfiring, loss of power. (engine damage)
Remedy : Check engine, ignition & mixture formation. New spark plugswith correct heat range.
14 Centre electrode melted away
Centre electrode melted away, ground electrode also severely attacked.
Cause : Overheating due to auto ignition, eg: due to over-advanced ignition timing, combustion deposits in combustion chamber, defective valves, defective ignition distributor, inadequate fuel quality.
Effect : Misfiring, loss of power, possibly engine damage. Overheated centre electrode may result in insulator nose cracking.
Remedy : Check engine, ignition & mixture formation. New spark plugs.
15 Partially melted electrodes
Cauliflower-like appearance of the electrodes. Possibly deposition of foreign matter.
Cause : overheating due to auto ignition, eg: due to over-advanced ignition timing, combustion deposits in combustion chamber, defective valves, defective ignition distributor, inadequate fuel quality.
Effect : Loss of power prior to complete failure. (engine damage).
Remedy : Check engine, ignition & mixture formation. New spark plugs.
16 Heavy wear on centre electrode
Cause : Recommended interval between sparkplug changes not complied with.
Effect : Misfiring, particularly when accelerating. (ignition voltage no longer sufficient for large electrode gap)
Poor starting performance
Remedy : New spark plugs.
17 Heavy wear on ground electrode
Cause : Aggressive fuel & oil additives.
Unfavorable influence of gas turbulence in the combustion chamber, possibly caused by deposits. Knocking. No overheating.
Unfavorable influence of gas turbulence in the combustion chamber, possibly caused by deposits. Knocking. No overheating.
Effect : Misfiring particularly when accelerating. (ignition voltage no longer sufficient for large electrode gap.)
Poor starting performance.
Remedy: new spark plugs.
18 Cracking of insulator nose
Cause: Mechanical damage due to impact, dropping or pressure on the centre electrode resulting from
incorrect handling. In marginal cases - especially after excessively long use - the insulator nose may crack due to deposits between the centre electrode and insulator nose, and due to corrosion of the centre electrode.
incorrect handling. In marginal cases - especially after excessively long use - the insulator nose may crack due to deposits between the centre electrode and insulator nose, and due to corrosion of the centre electrode.
Effect: Misfiring, sparkover at pointsnot reliably supplied with fresh mixture.
Remedy: New spark plugs.
Source – BOSCH Technical Resources.
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